Force fields affecting selected rigid body objects
This appears to be similar to the behavior described.



In this scene, the wood and metal cubes are affected by gravity, but only the metal cubes are attracted to the red cylinder.

In Blender 2.76, this can be accomplished using soft bodies instead of rigid bodies.



How to create this scene in Blender 2.76


Start with a new scene.
Add a plane, scale it. 
In the physics tab, add collisions. Set the soft body damping to 0.

Move the cube up. 
Go into edit mode. Select all vertices. 


Subdivide the mesh. This will ensure there are vertices and edges for collision detection. 
Create a new vertex group called 'Group'. Set the weight to 0. Assign all vertices to the vertex group. Settings later will use this to allow the soft bodies to move.


Go back to the object mode. Setup the soft body physics.


With the cube selected. Go to the physics tab. Select 'Soft Body'.
In the 'Soft Body Goal' panel, set the vertex group to 'Group'. This is the group assigned earlier.
In the 'Soft Body Edges' panel, select 'Stiff Quads', set the 'Push' value to 0.999, the 'Pull' value to 0.999, and bending to 0.100. This will make the soft body behave almost like a rigid body.
Select the 'Edge' and 'Face' collision options. This will help make the collisions more like a solid collision.


Test the simulation by pressing the play button on the timeline. You should see the cube fall to the plane, wobble a little, then stop. This indicates that the collisions are working and that the cube behaves like a solid.
Select 'Collision' physics. This will let the cubes collide with each other.

At this point, the cube is duplicated. One cube will be affected by force fields and the other will not. This will simulate the iron and wood.
Duplicate the cube. Rename the duplicate to 'Wood-Cube'.
Select the 'Wood-Cube', in the physics tab and the 'Soft Body Field Weights' panel, set the 'Force' to 0.000. This causes the 'Wood-Cube' to ignore force fields. 
 
Add an empty to the scene. Move the empty to the edge of the plane. In the physics tab, select force field. So the force field attracts objects, set the strength to a negative value. This examples will use -10.

This has all of the elements for the simulation! The 'Cube' will be attracted to the empty's force field. The 'Wood-Cube' will not be attracted, but will interact with 'Cube'. 
To troubleshoot, try different force strengths and clearing the soft body caches.




The file used to generate the animation above is located at